The ideal recording material for holography should have a spectral sensitivity well matched to available laser wavelengths, a linear transfer characteristic, high resolution, and low noise, should be indefinitely recyclable or relatively inexpensive. Hariharan reports on page 95 of his book P. Hariharan. Optical Holography. Principles, techniques, and applications. Chapter 7: “Practical recording materials,” 95-124. Cambridge University Press, 1996. P. 95 that “While several materials have been studied, none has been found so far that meets all these requirements”. The lack of available materials for phase holographs has stimulated the search for new approaches.
U.S. Pat. No. 6,586,141 issued on Jul. 1, 2003 to O. M. Efimov, L. B. Glebov, L. N. Glebova, V. I. Smirnov, owned by the same assignee as this application and having at least one common inventor described the process for production of high efficiency volume diffractive elements in photo-thermo-refractive glass which is incorporated herein by reference. This invention teaches how a photo-thermal process based on precipitation of dielectric microcrystals in the bulk of glass exposed to UV radiation and aged at elevated temperature can be used to record a high-efficiency volume phase hologram in glass because of a difference between refractive indices of exposed and unexposed areas of glass blank.
According to the present invention and references herein, the first step of the proposed process is the exposure of the glass sample to UV radiation, which produces ionization of a cerium ion. The electrons released from cerium are then trapped by a silver ion. As a result, silver is converted from a positive ion to a neutral atom. This stage corresponds to a latent image formation and no significant coloration or refractive index change occurs. The next step is thermal development. This development process includes two stages. The first involves the high diffusion rate silver atoms possess in silicate glasses. This diffusion leads to creation of tiny silver containing particles at relatively low temperatures in a range of approximately 450-500° C. A number of silver containing particles arise in exposed regions of glass after aging at elevated temperatures. These silver containing particles serve as the nucleation centers for sodium and fluorine ion precipitation and cubic sodium fluoride crystal growth occurs at temperatures between 500° C. and 550° C. Interaction of crystalline phase with glass matrix at elevated temperatures results in decreasing of refractive index in exposed areas compare to that in unexposed ones. This phenomenon was named the “photo-thermo-refractive” (PTR) process. Glasses, which possess such properties, were called “photo-thermo-refractive” (PTR) glasses.
Conditions of glass technology, exposure and development were found in that work to create volume holographic gratings (Bragg gratings) with relative diffraction efficiency exceeding 97%. The maximum recorded spatial frequency was about 10,000 mm−1. These gratings were stable up to about 400° C. The photosensitivity (difference of refractive indices between exposed and unexposed areas up to 10−3) was found in the range of several hundred mJ/cm2 at a helium-cadmium laser wavelength (about 325 nm). The absorption band of Ce3+, which is used for photo-ionization, has maximum near 300 nm and a long wavelength tale of up to 350 nm. This means that several commercial lasers such as N2, Ar, He—Cd, etc., emitting in this area can be used for recording. Once developed, holograms in PTR glass were not destroyed by further exposure to visible light. These properties of PTR holographic elements resulted in wide application of this technology for different laser systems operating in visible and near IR spectral regions.
However, many applications of holographic optical elements require high efficiency diffractive optical elements with narrower bandpass. Such elements, if used as selective components in lasers or in photo-receiving devices, can dramatically improve the performances of laser systems. However, high efficiency diffractive optical elements are limited to bandwidth higher than 30-40 μm due to limitation of PTR glass properties and recording system.
As already mentioned above, many solutions have been proposed to manufacture ultra-narrowband filters. H. A. Macleod, Thin-Film Optical Filter (Macmillan), third ed., Institute of Physics Pub., New York, 2001, pp. 37-53 teaches how to combine regular dielectric mirrors in order to create a Fabry-Perot cavity. However, such a filter requires the deposition of multi-layer coatings, the polishing of very high quality optical windows in order to produce ultra narrow bandpass associated with high throughput.
In J. Lumeau, M. Cathelinaud, J. Bittebierre and M. Lequime, “Ultra-narrow bandpass hybrid filter with wide rejection band”, Applied Optics 45 (7) 1328-1332 (2006), it was proposed to replace one of the mirrors of the Fabry-Perot cavity with a fiber Bragg grating. That way it is possible to select only one resonance of the Fabry-Perot cavity due to the spectral selectivity of the fiber Bragg grating and therefore to improve rejection. However, the experimental demonstration was only carried out in wave guided configuration.
In Y. O. Barmenkov, D. Zalvidea, S. Torres-Peiró, J. L. Cruz, and M. V. Andrés, Effective length of short Fabry-Perot cavity formed by uniform fiber Bragg gratings, Optics Express 14 (14) 6394 (2006), the co-inventors proposed replacing of both dielectric mirrors with two fiber Bragg gratings. In this case they showed that ultra-narrow bandpass filter can be fabricated. However, fabricated filters were also limited to the guided configuration and the method to implement such a filter did not include any issue regarding alignment of the two fiber Bragg gratings since they were self aligned inside the fiber.
In J. Lumeau, V. Smirnov, and L. B. Glebov, “Phase-shifted volume Bragg gratings in photo-thermo-refractive glass”, Proceeding of SPIE 6890, paper 68900A, (2008), authors demonstrated that two volume reflecting Bragg gratings recorded in PTR glass can be coherently combined in air in order to form a Fabry-Perot cavity. It was demonstrated that an ultra-narrowband filter with high throughput can be achieved, proving the ability of PTR glass to be used in such a resonant structure. However, the filter demonstrated was not monolithic and therefore not stable and not suitable for practical optical applications.
This problem was overcome in fibers by recording two superimposed fiber Bragg gratings (FBGs) with different periods within the same area of the fiber to obtain a Moiré Fiber Bragg Grating that allows obtaining a filter with properties similar to the one described and shown by a co-inventor in J. Lumeau, V. Smirnov, and L. B. Glebov, “Phase-shifted volume Bragg gratings in photo-thermo-refractive glass”, Proceeding of SPIE 6890, paper 68900A, (2008), but monolithic and in fiber described in R. Kashyap, “Fiber Bragg Gratings”, Academic Press; 1st edition (May 15, 1999). Such a structure was widely investigated in L. R. Chen, H. S. Loka, D. J. F. Cooper, P. W. E. Smith, R. Tam and X. Gu, “Fabrication of transmission filters with single or multiple flattened passbands based on chirped Moiré gratings”, Electronics Letters 35 (7) P. 584-585 (1999). However, due to the unavailability of bulk photosensitive materials, no investigation of bulk Moiré Fiber Bragg Gratings was performed.
What is needed is methods and systems to combine a Fabry-Perot cavity from volume Bragg gratings (VBGs) within one structure, and thus to form a Fabry-Perot filter which cavity and mirrors are composed by two superimposed VBGs. This invention teaches how to extend the technology of producing high efficiency diffractive optical elements in photosensitive material, e.g. PTR glass, to the fabrication of diffractive optical elements with narrower bandpass and apodized diffractive optical elements. Applications of those elements in military optical systems (laser radars, tracking systems, high-power lasers, etc.), optical communications (transmitters and receivers, WDM filters, etc.) and other markets should be extremely beneficial.